Prostate cancer is the most frequently diagnosed cancer and the second leading cause of male cancer death in North America and northern Europe. Early detection of prostate cancer using a serum test for prostate-specific antigen (PSA) has dramatically improved the treatment of the disease (Oesterling (1992) J. Am. Med. Assoc. 267:2236-2238). Treatment of prostate cancer consists largely of surgical prostatectomy, radiation therapy, androgen ablation therapy and chemotherapy. Although many prostate cancer patients are effectively treated, the current therapies can all induce serious side effects which diminish quality of life. Patients who present with metastatic disease are most often treated with androgen-ablation therapy. Hormone blockade results in significant regression of the tumor. However, this treatment rarely cures the patient and invariably results in progression to androgen-independent disease, which is incurable. Afrin and Stuart (1994) J.S.C. Med. Assoc. 90:231-236.
The identification of novel therapeutic targets and diagnostic markers is essential for improving the current treatment of prostate cancer patients. Recent advances in molecular medicine have increased the interest in tumor-specific cell surface antigens that could serve as targets for various immunotherapeutic or small molecule strategies. Antigens suitable for immunotherapeutic strategies should be highly expressed in cancer tissues and ideally not expressed in normal adult tissues. Expression in tissues that are dispensable for life, however, may be tolerated. Examples of such antigens include Her2/neu and the B-cell antigen CD20. Humanized monoclonal antibodies directed to Her2/neu (Herceptin) are currently in use for the treatment of metastatic breast cancer. Ross and Fletcher (1998) Stem Cells 16:413-428. Similarly, anti-CD20 monoclonal antibodies (Rituxin) are used to effectively treat non-Hodgkin's lymphoma. Maloney, et al. (1997) Blood 90:2188-2195; Leget and Czuczman (1998) Curr. Opin. Oncol. 10:548-551.
Several potential immunotherapeutic targets have been identified for prostate cancer. They include prostate-specific membrane antigen (PSMA)(Israeli, et al. (1993) Cancer Res. 53:227-230), prostate stem cell antigen (PSCA; Reiter, et al. (1998) Proc. Natl. Acad. Sci. USA 95:1735-1740), and serpentine transmembrane epithelial antigen of the prostate (STEAP; Hubert, et al. (1999) Proc. Natl. Acad. Sci. USA 96:14529-14534). PSMA is a type II transmembrane hydrolase with significant homology to a rat neuropeptidase (Carter, et al. (1996) Proc. Natl. Acad. Sci. USA 93:749-753). Antibodies directed towards PSMA are currently being used to detect metastasized prostate cancer as the Prostascint Scan (Sodee, et al. (1996) Clin. Nucl. Med. 21:759-767) and are also being evaluated for treatment of advanced disease (Gregorakis, et al. (1998) Semin. Urol. Oncol. 16:2-12; Liu, et al. (1998) Cancer Res. 58:4055-4060; Murphy, et al. (1998) J. Urol. 160:2396-2401). In a study on bone metastasis of prostate cancer, only 8 out of 18 patient samples expressed PSMA (Silver, et al. (1997) Clin. Cancer Res. 3:81-85). Therefore, it is clear that other targets need to be identified to manage metastasized disease. PSCA is a member of the Thy-1/Ly-6 family of glycosylphosphatidylinositol-linked plasma membrane proteins (Reiter, et al. (1998) Proc. Natl. Acad. Sci. USA 95:1735-1740). Immunohistochemical data shows that PSCA is up-regulated in the majority of prostate cancer epithelia and is also detected in bone metastasis (Gu, et al. (2000) Oncogene 19:1288-1296). Recent work shows that antibodies directed to PSCA can prevent metastatic spread of prostate cancer in a mouse model (Saffran, et al. (2001) Proc. Natl. Acad. Sci. USA 98:2658-2663). STEAP is a multi-transmembrane prostate-specific protein that may function as a channel or transporter protein (Hubert, et al. (1999) Proc. Natl. Acad. Sci. USA 96:14529-14534). Its protein expression is specific to the basolateral membranes of normal prostate and prostate cancer epithelia. STEAP expression was most highly concentrated at cell-cell boundaries, implying a potential function in intercellular communication. Therapeutic monoclonal antibodies have so far not been reported for STEAP.